How To Calculate Battery Run Time

There are just too many questions you will wonder when designing your device with a battery inside it.

The boss just want a cheap and small battery with it, but with no further information on how long does the end customer wants, how small can it be.

in this article, we will show you:

1 how to calculate the run time of a specific battery?
2 how much capacity can I get given a specific battery size requirement?
3 Online Lipo battery Size-Capacity Calculator
4 How to convert Watts to Amps or Amps to Watts or Volts to Watts

Ready for your battery design?
Let’s go.

In the ideal/theoretical case, the time would be Time(H) = Capacity(Ah)/Current(A).

If the capacity is given in amp-hours and current in amps, time will be in hours (charging or discharging).

Feel Confused?

Let’s see an actual case, 10 Ah battery delivering 1A, would last 10 hours. Or if delivering 10A, it would last for only 1 hour, or if delivering 5A, it would last only for 2 hours.

In other words, you can have “any time” as long as when you multiply it by the current, you get 10Ah (the battery capacity).

It is that simple.

So for a 18650 2500mAh(2.5Ah) battery with a device that draws 500mA(0.5A) you have:

2.5Ah/0.5A=5 Hours

Please take note that most batteries, especially those with circuits, will not work down to 0 Volts as a power supply (if it goes to zero, it will have shorter battery life, or even become dead battery if not charged in time), that’s to say,your circuit will stop working at a set voltage before the battery is fully drained.

We, therefore, will need to times 0.8-0.9 for the calculation:

that’s 2.5Ah/0.5A*0.9=4.5 Hours

What if you know Watts only, you will notice that every device use watt to determine it’s main specifications.

A 5 Watt bulb, 20W Laptop, 100W Motor, 200W Solar Street Light, you just name it.

In Theory, that’s:

Discharging Time=Battery Capacity*Battery Volt/Device Watt.

Say, 5AH*3.7V/10 Watts = 1.85 hrs

With 90% Power efficiency for Li-ion/LiPo batteries. Then
Discharging Time=Battery Capacity * Battery Volt*0.9 / Device Watt

5Ah*3.7V*0.9/10W = 1.66 hours

Let’s explain with more examples:

for a 1800mAH 3.7v 18650 battery to power a 3.7V 10W digital device, how to calculate the running time?

for 3.7V 10W device,working current would be 10÷3.7 = 2.7027A = 2702.7 mA
In theory that’s: 1800mAh ÷ 2702.7 mA = 0.666 h = 40 min
In reality that’s: 1800mAh ÷ 2702.7 mA*0.9 = 0.599h = 36 min

Notes: 1A=1000mA   mA is current, mAh is Capacity

Or you can use 3.7V*1.8Ah(1800mAh)*0.9/10W=0.599h=36min

Another example: 12V 60Ah battery pack to power 220V 100W light
Working time: 12V*60Ah*0.9/100W=6.48 H

How can calculate Battery capacity given a specific size limitation?

the battery usually comes with a pouch or cylindrical form.
in cylindrical form there is capacity listed for each size:

like for lithium 18650 battery: 2000mAh, 2600mAh,3000mAh, 3350mAh,4050mAh
for lifepo4 26650 battery:2500mAh, 3000mAh,3500mAh

if thickness allows, cylindrical battery cells will always be your first consideration on the table. They are on the shelf and therefore has short lead time and small MOQ.

for thickness less then 18mm, you will need to consider pouch lipo cells, and actually, it just comes with so many types of design, so it’s better to get a general idea of what capacity can you get for a given size.

The size of Lithium polymer battery has a great impact on its capacity, that’s the thickness, width, and length of a battery. The material and Production technology will have an effect on the lipo cell capacity.

The actual capacity will be very complex, but luckily we have a simple and fast calculate formula.

Capacity= Thickness*Width*Length*K

K= mah/mm³, which is a parameter that ranges from 0.07-0.12. for a general calculation, we will set it to 0.1

take 103450 battery cell for example (T=10mm, W=34mm, L=50mm)
that’s 10*34*50*0.1=1700, in reality, it’s about 1800mAh or more.

what about 603450(also called 063450 603450LP)?
that’s 6*34*50*0.1=1020 in actual it will be about 1050mAh

Online Size-Capacity Calculator

capacity less than 500-2000mAh

Thickness (mm)
Width (mm)
Length (mm)
Capacity (mAh)

capacity from 2000mah to 10000mAh

Thickness (mm)
Width (mm)
Length (mm)
Capacity (mAh)


1 the calculator does not apply to capacity less than 500mAh or 2mm less in thickness cells
2 If the capacity is not within the stated capacity range, try the other calculator
3 the capacity if for your reference, given the size, thickness, width, material, voltage, the capacity will still vary greatly from each other.

How to convert Watts to Amps or Amps to Watts or Volts to Watts?

You cannot convert watts to amps, HOWEVER, if you have at least two of the following three: amps, volts or watts then the missing one can be calculated.

The Following Equations can be used to convert between amps, volts, and watts.

Convert Watts to Amps
Convert Amps to Watts
Convert Watts to Volts
Convert Volts to Watts
Convert Volts to Amps
Convert Amps to Volts

Converting Watts to Amps
Basic equation: Amps = Watts/Volts
For example: 12 Watts/3 Volts = 4 Amp

Converting Amps to Watts
Basic equation Watts = Amps * Volts
For example   2amp * 100 volts = 200 watts

Converting Watts to Volts
Basic equation: Volts = Watts/Amps
For example 100 watts/10 amps = 10 volts

Converting Volts to Watts
Basic equation: Watts = Amps * Volts
For example 3amps *5 volts = 15 watts

Converting Volts to Amps at a fixed wattage
Basic equation: Amps = Watts/Volts
For example 100 watts/10 volts = 10 amps

Converting Amps to Volts at a fixed wattage
Basic equation: Volts = Watts/Amps
For Example: 48 watts/8 Amps=6Volts

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